I recently went on a highway drive in my 2006 f150 4x4, and after about 250 km, a knock............knock............knock came in the truck. I was cruising at about 110 km per hour at the time. I pulled over and shut the truck off. I started it back up again and continued my journey. It hasn't happened since. My thought was a vaccuuming problem with the 4x4, but I'm not real mechanically inclined.